Elastomeric diaphragms are widely used in many applications. The resilience of the diaphragms permits them to change in shape in response to, for example, an imbalance in fluid pressures applied to opposite sides of the diaphragm. A diaphragm may have any shape. The diaphragms of principal interest for the invention comprise an elastomeric material that is constrained at its periphery so that differential forces can be applied, causing the diaphragm to be at least locally distended and/or displaced in response to an imbalance in pressures applied to opposite sides of the diaphragm. The diaphragm isolates the pressure applying medium on one side of the diaphragm from the medium on the other side. A diaphragm may convert a fluid pressure into a mechanical force that can be employed to actuate some other element in response to the diaphragm displacement. Likewise, a mechanical force applied to a diaphragm by a plunger or other mechanical element causes an increase in fluid pressure on an opposite side of the diaphragm. The transferred pressure can be applied in many ways, for example, in a pump in which the diaphragm isolates the mechanical driving element from a pumped fluid.
Although the simplest diaphragm configurations are planar, elastomeric diaphragms are not restricted to such simple shapes. For example, a diaphragm may include a protruding shape for receiving a mechanical plunger or for responding in a non-linear way to a fluid or mechanical force. The walls of a protruding portion of a diaphragm may engage a mechanical plunger so that the configuration of the diaphragm changes as the position of the plunger changes in response to applied forces. A diaphragm engaging a plunger and following its movement through a circumferential fold can be considered to "roll". The circumferential fold in the diaphragm changes in position with the position of the plunger engaging the diaphragm. An example of such a rolling diaphragm in two different positions is illustrated in FIGS. 1 and 2 of U.S. Pat. No. 4,887,429, which is incorporated by reference.
The repeated flexing of a diaphragm in response to changes in pressure can gradually weaken a diaphragm. Moreover, each elastomer used in a diaphragm has an elastic limit and limited strength. The strength of a diaphragm may be changed by altering the thickness of a particular elastomer or by selecting a different elastomer. However, changing thickness may not be permissible in some applications where space is limited and diaphragm thickness is critical. Further, changing elastomer thicknesses or changing elastomers causes changes in elastic characteristics, such as stiffness, that affect the suitability of a diaphragm in a particular application.
One technique for increasing the tensile strength of elastomers, such as rubbers, beyond their inherent tensile strengths is the embedding of a reinforcing fabric within the elastomer. The conventional reinforcing fabrics have woven fibers or threads, usually woven biaxially, i.e., in two orthogonal directions. Fabrics conventionally used for such reinforcements include nylon and polyester.
A reinforcing fabric may be embedded within a planar elastomeric diaphragm or in a non-planar diaphragm, such as the high aspect ratio diaphragm illustrated in FIG. 1. The diaphragm of FIG. 1 has a peripheral flange 1 from which a conical part 3 of the diaphragm projects. The conical part 3 includes a relatively planar end surface 5 generally parallel to the flange 1. Together, the conical part 3 and the end surface 5 in the illustrated diaphragm describe major parts of the surface of a frustrum of a cone. As used here, the term high aspect ratio diaphragm means a non-planar diaphragm having a permanent projection, like the conical part 3 of FIG. 1, extending outwardly from a flange or other mounting part so that, when not stressed by applied pressure, the diaphragm does not lie in a single plane.
In a planar diaphragm, the reinforcing fabric is distorted only when the diaphragm is distorted by an applied pressure. In a high aspect ratio diaphragm, such as illustrated in FIG. 1, the reinforcing fabric is distorted from the intersection of the end surface 5 and the conical part 3 to the intersection of the conical part 3 and the flange 1. This distortion creates four regions, essentially at lines equally spaced around the conical part 3 of the diaphragm, where the yarn pattern of the fabric is particularly irregular. This phenomenon is called "four-cornering" and often results in excessive wear along those four lines as the diaphragm repeatedly flexes in response to applied forces.
Fabric reinforced diaphragms are typically made by clamping the reinforcing fabric and supplying a molten or flowable semi-solid elastomeric material to embed the fabric. Alternatively, the fabric may be compression molded between elastomeric sheets. When a high aspect ratio diaphragm, for example, having the configuration shown in FIG. 1, is manufactured, the fabric is typically formed on a mandrel or other projection of a mold. An elastomeric sheet may be first placed in the mold and the fabric and another elastomeric sheet placed on top of the fabric. Alternatively, after placement of the fabric, a molten or semi-molten elastomer is supplied to embed the fabric. The placement of a conventional fabric over a projecting mandrel is an initial source of distortion of the biaxial arrangement of the weave and fill yarns or threads of the fabric. It is this distortion of the fabric during manufacturing that leads to the "four-cornering" effect.
The fabric in a reinforced diaphragm inherently has a much lower elasticity than the elastomer, resulting in significant and even extreme localized compression of the elastomer when a diaphragm having the configuration shown in FIG. 1 "rolls". In that operation, because of an applied mechanical force, the planar end surface 5 moves transversely to that surface and includes a circumferential fold where the diaphragm has a 180.degree. bend or turn. The location of this bend on the conical part 3 changes as the diaphragm rolls. This bend significantly increases the frictional forces generated when the diaphragm is actuated. Generally, elastomers subjected to repeated compression cycles are likely to fail, unlike the well known durability of rubber and other elastomers in tensile cycles. Particularly in diaphragms having the configuration shown in FIG. 1 and a relatively small end surface area, the compression, i.e., frictional forces, produced upon repeated flexing, i.e., rolling, of the diaphragm can substantially decrease the lifetime of the diaphragm.
In a generally planar diaphragm including a fabric reinforcement, similar problems can arise. Although the reinforcing fabric is not flexed when a planar diaphragm is at rest, because of the nature of the distortion of the diaphragm when distended, similar frictional forces attributable to a non-symmetrical distortion of the fabric and compression of the elastomer can arise. These compressive and frictional cycles can lead to premature failure of an elastomeric diaphragm.